Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Because solar cells provide a sustainable energy resource, their use is rapidly expanding. Solar cells can typically be categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells.
Monocrystalline silicon (c-Si), poly- or multi-crystalline silicon (poly-Si or mc-Si) and ribbon silicon are the materials used most commonly in forming the more traditional wafer-based solar cells. Solar cell modules derived from wafer-based solar cells often comprise a series of about 180 and about 240 μm thick self-supporting wafers (or cells) that are soldered together. Such a panel of solar cells, along with a layer of conductive paste and/or connecting wires deposited on its surface, may be referred to as a solar cell assembly and encapsulated by or sandwiched or laminated between polymeric encapsulants, which may be further sandwiched between two protective outer layers to form a weather resistant module. The protective outer layers may be formed of glass, metal sheets or films, or plastic sheets or films. In general, however, the outer layer that faces to the sunlight should be sufficiently transparent to allow photons reach the solar cells.
In the increasingly important alternative, thin film solar cells, the commonly used materials include amorphous silicon (a-Si), microcrystalline silicon (μc-Si), cadmium telluride (CdTe), copper indium selenide (CuInSe2 or CIS), copper indium/gallium diselenide (CuInxGa(1-x)Se2 or CIGS), light absorbing dyes, organic semiconductors, etc. By way of example, thin film solar cells are described in U.S. Pat. Nos. 5,507,881; 5,512,107; 5,948,176; 5,994,163; 6,040,521; 6, 123,824; 6,137,048; 6,288,325; 6,258,620; 6,613,603; and 6,784,301; and U.S. Patent Publication Nos. 20070298590; 20070281090; 20070240759; 20070232057; 20070238285; 20070227578; 20070209699; 20070079866; 20080223436; and 20080271675. Thin film solar cells with a typical thickness of less than 2 μm are produced by depositing the semiconductor materials onto a substrate in multi-layers. Further, connecting wires, metal conductive coatings, and/or metal reflector films may be deposited over the surface of the thin film solar cells to constitute part of the thin film solar cell assembly. The substrate may be formed of glass or a flexible film and may also be referred to as superstrate in those modules in which it faces to the sunlight. Similarly to the wafer-based solar cell modules, the thin film solar cell assemblies are further encapsulated by or laminated or sandwiched between polymeric encapsulants, which are further laminated or sandwiched between protective outer layers. In certain solar cell modules, the thin film solar cell assembly may be only partially encapsulated by the encapsulant, so that only the side of the thin film solar cell assembly that is opposite from the substrate (or superstrate) is laminated to a polymeric encapsulant and then a protective outer layer. In such a construction, the thin film solar cell assembly is sandwiched between the substrate (or superstrate) and the encapsulant on the opposite side.
Within the solar cell modules, some components (such as connecting wires, conductive paste (used in wafer-based solar cell modules), conductive coatings (used in thin film solar cells) and back reflector films) may comprise oxidizable silver or a silver alloy. When in contact with an oxidizable silver component, an encapsulant comprising poly(vinyl butyral) (PVB) tends to discolor over time. Discoloration is not desirable in the photovoltaic industry, because it decreases the transmission of light, and because it may be considered aesthetically unpleasing. Thus, there is a need to develop a PVB composition that is useful as an encapsulant material for solar cell modules and that has improved resistance to discoloration when in prolonged contact with oxidizable silver components.